Watermarking of audio signals intends to manipulate the audio signal in a way that the changes in the audio content cannot be recognised by the human auditory system. Most audio watermarking technologies add to the original audio signal a spread spectrum signal covering the whole frequency spectrum of the audio signal, or insert into the original audio signal one or more carriers which are modulated with a spread spectrum signal. There are many possibilities of watermarking to a more or less audible degree, and in a more or less robust way. The currently most prominent technology uses a psycho-acoustically shaped spread spectrum, see for instance WO-A-97/33391 and U.S. Pat. No. 6,061,793. This technology offers a good compromise between audibility and robustness, although its robustness is not optimum.
In an other technology the encoded data, i.e. the watermark, is hidden in the phase of the original audio signal by phase coding: W. Bender, D. Gruhl, N. Morimoto, A. Lu, “Techniques for Data Hiding”, IBM Systems Journal 35, Nos. 3&4, 1996, pp. 313-336.
A further technology is phase modulation:
S. S. Kuo, J. D. Johnston, W. Turin, S. R. Quackenbusch, “Covert Audio Watermarking using Perceptually Tuned Signal Independent Multiband Phase Modulation”, IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), May 2002, vol. 2, IEEE Press, pp. 1753-1756.